1. Field of the Invention
The present invention relates to a printing apparatus, and particularly to the configuration of a printing head that uses a set consisting of a plurality of printing element arrays (nozzle arrays), which correspond to color agents to be ejected, to form an image.
2. Description of the Related Art
At present, personal computers, word processors and facsimile machines are employed widely in offices and at home, and for such systems, printing apparatuses employing a variety of printing systems have been provided for the output of information as printed matter. Of all these printing apparatuses, however, those that employ ink jet systems provide more advantages, e.g., comparatively, they are easily compatible with colorization, they produce less noise during operation, they print high-quality images on various types of printing medium, and they are compactly made. Ink jet printing apparatuses are further classified, in accordance with differences in their printing operations, into serial types and full-line types. And for personal use, of these two types, there is widespread acceptance of serial type ink jet printing apparatuses, compact devices that are available at low cost.
As the use of such serial type ink jet printing apparatuses has spread, there has been an increasing demand for printing apparatuses that can output higher quality images at higher speeds. In response to this demand, various techniques have been developed.
For example, a printing apparatus has already been provided that employs light cyan and light magenta, which have lower agent concentration, in addition to the basic four colors of cyan, magenta, yellow and black, in order to suppress the granularity of a highlighted portion and to obtain high gradation. Furthermore, a method and an apparatus have also already been disclosed whereby orange, red, green or blue ink, which has a different hue than has the basic four colors, is loaded for use in printing. Generally, image quality can be improved by using many types of ink, or by appropriately adjusting the components of the ink. In addition, ink such as dye ink and pigment ink, for which the hues are the same but properties such as permeability and diffusion differ, may also be selectively employed.
One well known example set of inks that is employed is made up of the following six ink types, i.e., black dye ink, yellow dye ink, magenta and light magenta dye ink, and cyan and light cyan dye ink. This set of inks is especially appropriate when a high quality photographic image, obtained using a digital camera or a scanner, is to be output to a glossy printing medium. Another example set of inks is made up of the following four ink types, i.e., black pigment ink, yellow dye ink, magenta dye ink and cyan dye ink. This set is especially effective when a black image, such as a black character or a table for which sharpness is important, is to be printed on plain paper.
Another factor that influences image quality is the size of the dot formed on a printing medium. For example, in a highlighted portion, small dots are better suited for forming in order to suppress granularity of the printed portion, while in high density portions, large dots are better suited for forming in order to obtain an appropriate optical density. Thus, a printing head and a printing method have previously been disclosed that enable the printing of two sizes of dots, i.e., large and small dots. When individual pixels can be expressed at densities having multiple levels, high image gradation can be obtained.
When a plurality of types of ink are provided for individual pixels, to obtain a high quality image, ink dots are preferably not overlapped. Especially for cyan and magenta, their luminosity would be decreased due to dots overlapping each other. A technique has already been disclosed whereby these dots are formed so that, to the extent possible, they are separated in the same pixel. This technique is hereinafter referred to as a CM separation technique. Details of the CM separation technique have already been disclosed, for example, in Japanese Patent Application Laid-open No. 2003-116014.
When the above described technique has been realized for a serial type ink jet printing apparatus, various problems unique to the serial type have occurred. For example, for a color ink jet printing apparatus wherein cyan, magenta and yellow ink nozzles are arranged in parallel in the scanning direction of a printing head, an imaging problem called color banding has occurred due to the order in which ink is provided to a printing medium. In the case of a printing head for the above described arrangement, the order in which colored inks are provided to the printing medium is reversed between the forward path and the return path for scanning. That is, for example along the forward path, colored ink is provided to the printing medium in the order cyan, magenta and yellow, and along the return path, colored ink is provided in the order yellow, magenta and cyan. This difference in the printing order produces, more or less, a difference of hue on a printing medium. Therefore, as one result of the printing of an image having a uniform tone, the image areas printed along the forward path and the image areas printed along the return path are alternately arranged with different colors, which are developed, and there is considerable deterioration in the image quality.
To completely prevent this color banding problem, an image need merely be formed by scanning only along the forward path or only along the return path. However, compared with bi-directional scanning, the printing period required for such unidirectional scanning is greatly extended. Thus, to resolve this problem, a method has been disclosed whereby two nozzle arrays for the individual colors are symmetrically arranged on the printing head, on either side in the scanning direction, so that color banding is prevented and bidirectional scanning is performed (see, for example, Japanese Patent Application Laid-open No. 2001-96770 and No. 2001-96771).
In Japanese Patent Application Laid-open No. 2001-96770, a symmetrical printing head is disclosed wherein nozzle arrays are arranged in the order CMYYMC, for example, in the main scanning direction. According to the description given for this reference material, since ink of the same color is evenly ejected through two nozzle arrays during a single scanning, not only can color banding due to the printing order be prevented, but also discrepancies in ink ejection by the individual nozzles can be dispersed, so that, in appearance, they are not noticeable.
According to the invention disclosed in Japanese Patent Application Laid-open No. 2001-96771, nozzle arrays symmetrically arranged are employed evenly as in Japanese Patent Application Laid-open No. 2001-96770, and a plurality of dots of the same color are printed as a single pixel to provide multi-gradient printing. For this gradient printing, sorting of data for the right and left nozzle arrays is controlled in accordance with the gradation value to maintain equal frequencies for the use of the two nozzle arrays.
For an apparatus such as an ink jet printing apparatus that represents an image by arranging multiple dots, how to form dots on a printing medium in high accuracy of position is important in order to obtain a high quality image. However, in the above described serial type ink jet printing apparatus there are many weak points whereat mechanical errors tend to occur, e.g., accuracy errors tend to occur during the moving of a carriage whereon a printing head is mounted, during the mounting of the printing head on the carriage and during the conveying of a printing medium. So, the occurrence of such mechanical errors cannot be prevented completely. When they become excessive, they can be identified easily and adversely affect the appearance of an image. It is possible, however, to perform calculations that can improve either the configuration of a printing head or a printing method, and to cause effects that adversely affect an image to become less noticeable. That is, the next issue is how to handle various mechanical errors so, though they may affect the appearance of an image, they become less noticeable.
However, previously, satisfactory studies have not been performed to determine how the above described mechanical errors actually affect an image when a conventional configuration includes large and small ink discharge orifices or a symmetrical printing head, or when a new printing method, such as the CM separation technique, is employed. Furthermore,there has not been a satisfactory study performed to determine an appropriate printing head and an appropriate printing method for preventing such effects.
Through studies performed by the present inventors, it was confirmed that new image effects occurred when a symmetrical printing head was mounted so it was inclined relative to the scanning direction. Specifically, for a CMYYMC symmetrical printing head, we found that the shifting was the greatest for cyan dots located the furthest outward, and that between dots, the distance shifted differed, depending on the set of nozzle arrays employed. The difference in the shifting distance caused cyclical color unevenness, and this was identified as a distinctive image affect. Further, it was also confirmed that a state wherein multiple nozzle arrays were arranged in a printing head influenced the degree to which image deterioration occurred due to the above described mechanical errors. As a result of intense study, performed employing a serial ink jet printing apparatus that carries out CM separation using large and small dots, the present inventors found a new optimal orifice arrangement for a printing head and an optimal printing method.